1. Field of the Invention
Embodiments of the present invention generally relate to methods and apparatus for impact drilling. Particularly, embodiments of the present invention relate to a drilling tool that impacts while simultaneously rotating a drill head, independent from the rotation of the drill string.
2. Description of the Related Art
A percussion method of drilling a well bore into an earthen formation, especially hard rock, involves a cyclic and spikelike impacting force rather than a steady pressing force imposed by the weight of the drill string. This percussive action produces a superior high rate of penetration versus the traditional drill-by-weight method.
By employing a percussion drilling tool, the drill head needs to be rotated so that the cutting elements mounted on its face come to contact with fresh rock formations during each subsequent strikes. Traditionally, this need is achieved by keying the drill head to the drill string so that the rotation of the drill string, provided by a rotary table mounted on the rig, and in the range of 20 to 40 rpm, is transferred to the drill head.
The percussion drilling tools are pneumatic devices connected to the end of a drill string. Highly compressed air is directed alternately into and out of two separate chambers. One chamber is positioned above a sliding body, commonly known as a piston, and the other chamber is positioned below the sliding body so that the air causes the body to accelerate up and down, reciprocating within the tool housing. During the tool operation, the drill head is kept in contact with the earth at the bottom of a well bore. As the sliding body is directed downward, it forcefully strikes the top of the drill head and causes the rock contacting the drill head to disintegrate. As stated above, it is desired to rotate the drill head to allow it to penetrate fresh rock during subsequent strikes from the sliding body. Although percussion drilling tools may afford faster penetration rates, the need to rotate the entire drill string takes away the ability to deviate the well bore trajectory in the desired direction.
To apply the requisite striking force that will break the rock formation, the reciprocating piston travels at a relatively high linear velocity, in the range of 300 to 400 inches per second. In methods that employ the kinetic energy of the axial motion of the piston to induce a rotational motion on the drill head, high velocity motion between contacting bodies may be involved. Moreover, torques of high magnitudes, in the range of 500 to 1,000 foot pounds under ideal conditions, and up to 3,000-4,000 foot pounds under adverse conditions, are required to rotate the drill head against frictional forces imposed by the formation and inevitably cause high contact stress at the surfaces adjacent to the piston and drill head. The combined effect of high contact velocity and high contact stress generates a great deal of friction and heat, resulting in severe galling damage at these contact surfaces.
In conventional drill-by weight method, the force that is used to press the drill head against the bottom of the formation, commonly called weight-on-bit, is typically between 20,000 to 50,000 pounds. In percussion drilling, since it is the impact force of the reciprocating piston against the drill head that breaks up the formation, this immense weight-on-bit is not needed. However, as the tool penetrates the formation, the drill head tends to slide out of the housing of the tool. If the drill string is not allowed to keep up with the drill head progression into the formation, the tool can enter into an “opening position” and stop cycling. Therefore, it is dependent on the skill of the operator to advance the drill string into the well bore quick enough to prevent the tool from opening.
On the contrary, however, if the weight of the drill string is not held back properly, the drill string can apply excessive weight onto the drill head. This is also undesirable since the extreme weight-on-bit dramatically increases the frictional torque necessary to rotate the drill head. The operator thus faces the difficult task of advancing the drill string, on the one hand, quick enough to prevent the tool from opening, and on the other, slow enough to avoid pressing the drill head too hard against the formation. The operator must hold back most of the drill string weight, yet strives to allow just enough force to keep the tool closed. Frictional drag created by contact between the drill string and the walls of the well bore exacerbates this dilemma.
Therefore, there is a need for a percussion drilling tool capable of rotating the drill head independently from the drill string, without the detrimental galling effects caused by motion under high contact stress at high velocity. There is also a need for providing a means with which the driller can rely on to advance the drill string into the well bore without pressing the drill head neither too hard nor too lightly against the formation.